Technologies for atomizing fluids and materials are important to a number of industries and to a wide range of applications. One particular application is the delivery of combustible fuels to spark-ignition engines. For this application it is desired to minimize the size of the resulting droplets, or to yield vaporization of the fuel. In fuel delivery systems it is understood that reduced droplet size leads to greater combustion efficiency, which, in turn leads to reduced waste and greater environmental performance.
One common technique for atomizing a liquid fuel is to employ an aspirating gas flow to break-up the liquid into droplets. This technique is employed by carburetors, which are still the predominant fuel delivery system used today for small combustion engines. Although these aspirating systems yield acceptable results for small gasoline powered spark ignition engines, the size of the droplets produced during atomization is still relatively large, and less than optimal for many fuels and more demanding applications. Improvements and modifications that would eliminate or reduce the size of drops provided by the aspiration technique have been suggested, such as screening and flow redirection, but these modification also reduce throughput and create waste problems. Moreover, neither of these improvements address an additional problem with the aspirating technique, which is that the aspirating gas, typically air, can dilute the fuel being delivered, reducing the concentration of the fuel delivered and reducing efficiency of subsequent combustion.
In light of these problems, a considerable amount of research has gone into developing techniques that provide greater atomization of liquid fuels, without requiring an aspirating gas. For example, ultrasonic and electrostatic atomization devices have been developed and employed to further reduce drop size and to eliminate, or reduce, the need for an aspirating gas flow. However, the results achieved with such systems have been mixed and such systems often fail to provide the desired high droplet velocities. Vaporization via heating has also been used, but this technique has resulted in distillation and problems such as fractionation, residue buildup, and decomposition.
By far the most successful alternative to the carburetor has been the fuel injector. Fuel injectors, although more complicated and expensive, provide direct, proportional fuel metering capability under electrical control. Automotive engines in the U.S. now use gasoline fuel injection of two primary types: 1) throttle body injection for the whole engine and 2) port fuel injection for each cylinder. The primary advantages of fuel injection are better specific power (lower fuel consumption per unit power generated) and far better integration with the engine control unit; this results in much lower emissions through better control under a variety of environmental and operating conditions. The control unit can then implement ever more complex and efficient control algorithms, integrating with more sensors to determine optimal settings for the simple control (timing and duration) of the fuel injector, thus further improving fuel economy and reducing exhaust emissions. Accordingly, the fuel injector allows the use of computerized control systems to help optimize performance.
However, fuel injectors efficiencies are still limited by the size of the drops that form the injector spray. Increasingly over the last few years, much experimentation and advanced design has been conducted on the mechanical configuration of fuel injectors to improve their spray/atomization characteristics while keeping their electrical controllability, which is their chief advantage. For example, there has been extensive research and experimentation with forced-air assisted atomization, micro-machining of injection plates, use of flow-swirling effects as well as other approaches. Although significant improvements have occurred, several hurdles still remain. For example, the spray/atomization characteristics of today's fuel injectors is still insufficient to allow for the use of low vapor pressure fuels, also know as heavy fuels, such as kerosene. To date, the new injector designs and processes do not achieve the very fine atomization and subsequent vaporization required for a successful operation as an spark ignition engine with a heavy fuel. The literature does report that one researcher has successfully cold started a small spark-ignition engine with a crude but effective kerosene vaporizing unit, however this technology is not very controllable nor packageable as a compact fuel injector. Nor is it clear that these systems can provide the very lean mixture that will ignite reliably and controllably at the high compressions ratios needed for the efficiencies demanded today. The approach taken by other researchers has been direct injection of heavy fuels into the engine cylinder. It is not clear however, that these attempts at direct injection vaporization of fuel will actually circumvent the problems of cold start and proper running of kerosene-fueled, spark-ignition engine at low ambient temperatures conditions. Moreover, present research in using heavy fuels with spark ignition engines has shown that the existing techniques fail to deliver fuel in a manner that will effectively ignite, causing the build up of liquid fuel in the engine and the eventual fouling of the spark plug.
Accordingly, there is a need in the art for a fuel delivery system that provides improved vaporization capability to allow for the delivery of conventional low vapor pressure liquid fuels and that can be packaged and engineered as both a direct (in cylinder) or indirect (port/throttle body) fuel injector.
It is further desired to provide fuel delivery systems that can produce atomized volumes of fuel with sufficiently small droplet size to overcome the long-standing problems with cold-start when employing low vapor pressure fuels to fire small, lightweight, heavy fuel-fired, spark-ignition engines.